In mobile communication, orthogonal frequency division multiplexing (OFDM) is attracting attention as one of multicarrier transmission schemes capable of reducing intersymbol interference in the multipath environment. However, with an OFDM scheme using sub-carrier modulation, the multicarrier-modulated signal, namely, the output of inverse fast Fourier transform (IFFT) has large peak amplitude as compared with the average level.
For this reason, peak to average power ratio (PAPR) increases, and nonlinear distortion is generated as illustrated in FIG. 1. This problem is an attribute of multicarrier modulation and caused by the following reason. When signal components of individually modulated multiple carriers are combined in phase, the adder output for a certain signal at a certain point of time become extremely high, and consequently, the combined signal has a large peak as compared with the average output level.
FIG. 2 is a graph showing an input/output characteristic of a typical transmission amplifier. As illustrated in FIG. 2, the region with a linear input/output characteristic is limited. The signal component beyond this linear region is clipped, and a signal is output with a distorted peak. This causes degradation of transmission quality and increases an out-of-band radiation power level. It is known that if the linear region is expanded, the amplification efficiency falls. Accordingly, it is desired for the amplitude (power level) distribution of a transmission signal to contain as little signal component with large amplitude as compared with the average as possible.
FIG. 3 and FIG. 4 are block diagrams of a typical OFDM transmitter and a typical OFDM receiver, respectively. In the OFDM transmitter shown in FIG. 3, a signal generator 1001 performs error correction encoding, interleaving, and symbol mapping on an input information bit sequence to produce transmission symbols. The transmission symbols are subjected to serial-to-parallel conversion at the serial-to-parallel (S/P) converter 1002 and converted into multiple parallel signal sequences. The S/P converted signal is subjected to inverse fast Fourier transform at IFFF unit 1003. The signal is further subjected to parallel-to-serial conversion at the parallel-to-serial (P/S) convert converter 1004, and converted into a signal sequence, as illustrated in FIG. 5. Then, guard intervals are added by the guard interval (GI) adding unit 1005, amplified at the power amplifier 1006, and finally transmitted as an OFDM signal by radio.
On the other hand, at the OFDM receiver shown in FIG. 4, the guard interval is removed from the received signal at the guard interval removing unit 2001. Then, the received signal is subjected to serial-to-parallel conversion at S/P converter 2002, fast Fourier transform at the FFT unit 2003, and parallel-to-serial conversion at P/S converter 2004, as illustrated in FIG. 5. Then, the received OFDM signal is detected to acquire the transmitted information.
To solve the above-described PAPR (Peak to Average Power Ratio) issue in an OFDM transmission scheme, various methods for reducing the peak amplitude (power level) are proposed. Such proposals include a frequency domain interleaving method, a clipping filtering method (See, for example, X. Li and L. J. Cimini, “Effects of Clipping and Filtering on the Performance of OFDM”, IEEE Commun. Lett., Vol. 2, No. 5, pp. 131-133, May, 1998), a partial transmit sequence (PTS) method (See, for example, L. J Cimini and N. R. Sollenberger, “Peak-to-Average Power Ratio Reduction of an OFDM Signal Using Partial Transmit Sequences”, IEEE Commun. Lett., Vol. 4, No. 3, pp. 86-88, March, 2000), and a cyclic shift sequence (CSS) method (See, for example, G. Hill and M. Faulkner, “Cyclic Shifting and Time Inversion of Partial Transmit Sequences to Reduce the Peak-to-Average Ratio in OFDM”, PIMRC 2000, Vol. 2, pp. 1256-1259, Sep. 2000).
In addition, to improve the receiving characteristic in OFDM transmission when a non-linear transmission amplifier is used, a PTS method using a minimum clipping power loss scheme (MCPLS) is proposed to minimize the power loss clipped by a transmission amplifier (See, for example, Xia Lei, Youxi Tang, Shaoqian Li, “A Minimum Clipping Power Loss Scheme for Mitigating the Clipping Noise in OFDM”, GLOBECOM 2003, IEEE, Vol. 1, pp. 6-9, Dec. 2003). The MCPLS is also applicable to a cyclic shifting sequence (CSS) method.
FIG. 6 is a block diagram of an OFDM transmitter employing MCPLS cyclic shifting sequence. In this example, subcarriers are grouped into two blocks. FIG. 7 is a block diagram of a dividing IFFT unit 1013 used in the OFDM transmitter shown in FIG. 6 and configured to divide eight subcarriers into two blocks.
The dividing IFFF unit 1013 produces two time signal sequences, namely, a first subsequence containing signal components of subcarriers 0 through 3, and a second subsequence containing signal components of subcarriers 4 through 7. In ordinary OFDM signal generation, the two groups of time signal sequences are added and the combined signal is output as a transmission signal. However, with CSS, phase rotation is applied to a portion of time signal sequences, and then added to the other portion of the time signal sequences. In addition, in CSS, cyclic shifting is applied to a portion of the time signal sequences at the cyclic shifting unit 1012, and added to the other portion of the time signal sequences, as illustrated in FIG. 6. By preparing multiple levels of cyclic shifting, multiple candidates are produced for a same transmission signal sequence. The PAPR reduction control unit 1011 using MCPLS detects a total of exceeding power over the reference level for each of the time signal sequences output from the dividing IFFF unit 1013, and selects a signal sequence with the minimum exceeding power as a target signal sequence to be transmitted.
In a partial transmit sequence (PTS) scheme, an appropriate set of phase rotation values determined for the respective subcarriers in advance is selected from multiple sets, and the selected set of phase rotations is used to rotate the phase of each of the subcarriers before signal modulation in order to reduce the peak to average power ratio (See, for example, S. H. Muller and J. B. Huber, “A Novel Peak Power Reduction Scheme for OFDM”, Proc. of PIMRC '97, pp. 1090-1094, 1997; and G. R. Hill, Faulkner, and J. Singh, “Deducing the Peak-to-Average Power Ratio in OFDM by Cyclically Shifting Partial Transmit Sequences”, Electronics Letters, Vol. 36, No. 6, 16th Mar., 2000).
FIG. 8 and FIG. 9 are block diagrams of an OFDM transmitter and an OFDM receiver, respectively, which employ a partial transmit sequence (PTS) scheme. In FIG. 8, the signal sequences generated by the signal generator 1001 is divided into two groups by the dividing unit 1031 of the dividing IFFF unit 1023. Serial-to-parallel conversion and inverse fast Fourier transform are performed on each of the divided groups.
Each of the IFFT units 1033 has N input/output points to receive N/2 signals from the S/T converter 1032 and N/2 null symbols. The phase rotation control unit 1021 determines an appropriate set of phase rotations or weighting values (θ1, θ2, . . . ), and one of the elements of the set is applied to the multipliers 1034 in common. In this manner, the outputs of the IFFT units 1033-1 and 1033-2 are combined under appropriate weighting at adders 1035.
The combined signal sequences are subjected to parallel-to-serial conversion at P/S unit 1004, a guard interval is added to the serial signal at guard interval unit 1005, and the signal is finally transmitted from the antenna.
In the receiving end, the phase rotation is adjusted when the signal is demodulated, as illustrated in FIG. 9.
However, in the above-described prior art techniques, if the number of groups of the divided subcarriers and the number of cyclic shifting patterns increase, the candidates of the transmitted signal increase exponentially, and the computational workload is extremely increased.